Re: [md PATCH 0/5] Stop using bi_phys_segments as a counter

[NeilBrown <neilb@suse.com>]

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On Tue, Nov 22 2016, Shaohua Li wrote:

> On Mon, Nov 21, 2016 at 12:19:43PM +1100, Neil Brown wrote:
>> There are 2 problems with using bi_phys_segments as a counter
>> 1/ we only use 16bits, which limits bios to 256M
>> 2/ it is poor form to reuse a field like this.  It interferes
>>    with other changes to bios.
>> 
>> We need to clean up a few things before we can change the use the
>> counter which is now available inside a bio.
>> 
>> I have only tested this lightly.  More review and testing would be
>> appreciated.
>
> So without the accounting, we:
> - don't do bio completion trace

Yes, but hopefully that will be added back to bio_endio() soon.

> - call md_write_start/md_write_end excessively, which involves atomic operation.

raid5_inc_bio_active_stripes() did an atomic operation.  I don't think
there is a net increase in the number of atomic operations.

>
> Not big problems. But we are actually reusing __bi_remaining, I'm wondering why
> we not explicitly reuse it. Eg, adds bio_dec_remaining_return() and uses it
> like raid5_dec_bi_active_stripes.

Because using it exactly the same way that other places use it leads to
fewer surprises, now or later.
And I think that the effort to rearrange the code so that we could just
call bio_endio() brought real improvements in code clarity and
simplicity.

Thanks,
NeilBrown

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